Wires of metals such as tungsten, copper, iron, molybdenum, stainless steel, and the like, are produced by drawing the metals through dies of hard materials, such as diamond, tungsten carbide, and the like. Single crystal diamond dies are especially useful, but they can be difficult to fabricate, tend to chip easily, and often fail catastrophically because of the extreme pressures involved and because they are relatively easily cleavable. Tungsten carbide dies are less expensive, but have short die lives and are much less harder than diamond. A new family of dies which avoid these problems of single crystal diamonds and tungsten carbide comprise microporous masses compacted from tiny crystals of natural or synthesized diamonds or from crystals of cubic boron nitride. The compacts are formed by sintering, cementing and similar processed, and the technology involved in producing compacted diamond crystals of this type can be found, for example, in Strong, U.S. Pat. No. 3,407,445, Wentorf & Rocco, U.S. Pat. No. 3,831,428, Bovenkerk et al, U.S. Pat. No. 3,744,982 and U.S. Ser. No. 292,155, filed Sept. 25, 1972 by H. P. Bovenkerk and Glenn T. Malloy, entitled "Method of Making Diamond Compacts", and now abandoned, which describe high pressure reaction processes for the preparation of diamond compacts of predetermined shapes from shaped charges. Other useful disclosures by way of background may be found in Wentorf and Rocco, U.S. Pat. No. 3,745,623. Disclosure relating to the technology of formation of compacted bodies of finely divided cubic boron nitride crystals can be found in Wentorf, U.S. Pat. No. 3,233,988, Wentorf and Rocco, U.S. Pat. No. 3,743,489, and the said U.S. Pat. No. 3,744,982. Methods of forming polycrystalline compacts are also disclosed in Hall U.S. Pat. Nos. 3,829,544 and 3,816,085. The disclosures of these patents and the application are incorporated herein by reference. In especially preferred embodiments, there are provided composite wire drawing die constructions in which the die core comprises a compacted mass of diamond, cubic boron nitride, or a mixture thereof, and an outer jacket comprises a cemented carbide bonded directly to the core. The outer jacket provides compressive support to the die core. Moreover, it facilitates mounting the die in high strength metal rings, for example, because the expense of grinding the outer irregular surface of the die (as made) to a suitable shape is avoided.
The holes of wire drawing dies prepared from such compacts have a micro-rough structure, i.e., they contain tiny voids and/or irregularities between microcrystals.
The polycrystalline hard masses made via the teachings of Bovenkerk and Malloy, on the one hand, and Hall, on the other hand, which may be used for wire drawing dies generally contain both micro-voids/pores and softer inclusions distributed relatively uniformly throughout the polycrystalline mass. They are not flanked or jacketed by metal bonded carbide bonded directly to the polycrystalline mass as in composite wire drawing dies. In the latter, the polycrystalline mass of diamond and/or cubic boron nitride contains very few if any micro-voids/pores. The metal cemented carbide jacket provides a source of flowable metal, e.g., cobalt, at pressure and temperature conditions of preparation which sweeps or infiltrates through the polycrystalline mass and fills any voids/pores with metal.
In making any wire drawing die, however, a double-tapered hole is drilled (or otherwise formed in the compacting process), shaped and polished to a size required in the polycrystalline diamond and/or cubic boron nitride mass. A polycrystalline diamond or cubic boron nitride die hole surface is microscopically rough after polishing, and pockets in the wire drawing surface appear to be due to the following factors:
i. micro-voids/pores throughout the initial polycrystalline mass prior to drilling and, therefore, present on the hole surface after drilling; PA1 ii. softer metal or non-metallic inclusions along crystal grain boundaries or as particles in the initial polycrystalline mass where diamond-diamond bonding is not present are more easily eroded or polished away leaving a microporous and/or "channeled" hole surface; PA1 iii. diamond crystals have a high degree of hardness variation depending on crystallographic orientation and, therefore, a polycrystalline mass of randomly oriented diamond will have a rough surface due to variations in the ease of polish of individual crystals; and PA1 iv. micro-chipping of individual crystals occurring during polishing and resulting in a micro-rough hole surface.
In any event, because of these voids, surface irregularities and variations in the degree of hardness of individual microcrystals due to crystal orientation, the pierced or otherwise formed double tapered drawing aperture or throat does not polish to the same finish as can be obtained with mined single crystal diamonds. In fact, the throat of the die exhibits considerable porosity, e.g., ranging from 3 to 20% by volume. This porosity and surface irregularity due to variability in crystal hardness is responsible for production of wires with unsuitable surface finish and causes an undue amount of force to be expanded in the drawing operation, in comparison with single crystal dies, for example. It has now been discovered that if the pores in the compact which open into the throat of the die bore are densely packed with a solid lubricant, there is an unexpected improvement in performance of the die. The means for packing the pores with the selected lubricant are not critical, but one convenient way to do so is to draw a lubricant coated wire blank through the die. This causes transfer of the lubricant from the blank to the pores of the die bore surface by stripping it from the blank during passage through the die. Dense packing is insured because of the force exerted by the blank against the hole walls. If such a die then is used to draw wires in production, the force required to pull wires through it is substantially less than that required for microporous dies which have not been packed with lubricant in accordance with this discovery. Moreover, the surfaces of the wires drawn through such lubricant-packed compact dies is much smoother, and more comparable to those prepared by drawing through single crystal diamond dies.
One partial explanation for the large drawing force required with an unlubricated die is thought to be that the micro-voids tend to fill with metal shaved from the drawn wire, which voids when filled, in turn cause an increase in the drawing force due to metal to metal bonding or welding between the entrapped metal and the metal wire. The tendency of the voids to be filled with metal is thought to be reduced in accordance with this discovery.
In addition to reducing the drawing force and improving the surface appearance of the drawn wire, the lubricant-packed dies of this invention provide superior wear properties.
Also, the lubricant is thought to have a tendency to self-heal small chipped regions in the bore thereby reducing the tendency of the chipped region to be enlarged by additional chipping.